1. Field of the Invention
The present invention relates to a method for centrally setting data rate in a data transmission facility and to an associated device for centrally setting data rates in the data transmission facility.
2. Description of the Prior Art
In a known data transmission facility described in DE 299 08 608 U1, the data rate for transmission of data is set using special telegrams. In this case, all receivers are preset to a data rate in the data rate setting step and are thus ready to receive and evaluate special telegrams. The data rate for future data transmission is set with reference to the special telegram for the data rate setting. The content of the special telegram includes information regarding the data rate. This is always transmitted with a single, preset data rate for the purpose of setting the data rate. However, the setting of the data rate according to this described method has the critical disadvantage that for central data rate setting the central station must send out special telegrams. This data transmission facility does not permit compatibility with subscribers that have no data rate setting, since transmission is only assured if all subscribers are capable of evaluating the special telegram.
From DE 44 18 622 C2, a method is known for determining the transmission rate in a bus system, according to which the transmission rate corresponding to the bit length is selected from a table by bit length measurement. In this described method, a counter is started when bit edge detection is begun, and stopped again after the following bit edge. The counter value determined thereby represents a bit length that corresponds to a data rate. The counter value is compared with a standard table of reference bit lengths. If the measured bit length matches a value in the table, the data rate is set accordingly. The method presented here is highly unreliable for data transmission facilities because signal edges in data circuits may be created by, for example, electromagnetic interference. As a consequence, reliable data rate setting in an industrial environment where electromagnetic disturbances abound is not possible with this measurement method. Since the counter for bit length measurement returns a value that corresponds to a data rate even with predetermined deviations, the realization thereof involves a great deal of effort and must be clocked at a higher system speed. Otherwise, the measurement will become very inaccurate at higher data rates because of the system-conditioned sampling error.
In INTERBUS basics and practice, ISBN 3-7785-2471, 1998, a serial data transmission protocol according to the INTERBUS transmission method is described. Synchronization of the telegrams and the function of the status telegrams are described here. This description always assumes a fixed data rate, which must be preset on all subscribers. In the data transmission method described here, no provision is made to enable the data rate to be set or changed centrally for all subscribers.